1. Field of the Invention
This invention relates to a method of recovering and concentrating an aqueous N-methylmorpholine-N-oxide (NMMO) solution.
2. Description of the Related Art
NMMO (N-methylmorpholine-N-oxide) is a nontoxic organic solvent, and is used in lyocell filament production. Due to the high cost of NMMO, NMMO is usually recovered from and reused in the lyocell filament production.
A conventional method of producing lyocell filaments includes steps of: dissolving cellulose pulp in an aqueous N-methylmorpholine-N-oxide (NMMO) solution so as to obtain a spinning solution, and manufacturing lyocell filaments from the spinning solution by a spinning process. When the spinning process is conducted, the spinning solution is extruded through a spinning funnel into a coagulating bath. In the coagulating bath, the NMMO is extracted from the extruded spinning solution to water to obtain an aqueous NMMO waste solution, such that the lyocell filaments are formed. Subsequently, the lyocell filaments are washed with water to remove residual NMMO (washing step). The aqueous NMMO waste solutions obtained from the coagulating bath and the washing step are collected, recovered and concentrated for recycling.
FIG. 1 illustrates a triple effect evaporator that is used in a method for recovering and concentrating the NMMO solution. The triple effect evaporator includes serially connected first, second and third effect evaporators 11, 12, 13 and a condenser 14. The aqueous NMMO solution is sequentially concentrated using the first, second and third effect evaporators 11, 12, 13 to obtain first, second and third concentrated aqueous NMMO solutions. In the conventional method of recovering and concentrating the aqueous NMMO solution, the evaporators 11, 12, 13 have to be maintained at an elevated temperature so as to evaporate the NMMO solution fed thereinto. The water vapor thus produced from the first effect evaporator 11 is introduced into the second effect evaporator 12 for heating the solution therein. In a similar manner, the water vapor thus produced from the second effect evaporator 12 is introduced into the third effect evaporator 13 for heating the solution therein. The water vapor produced from the third effect evaporator 13 is then cooled and liquefied by the condenser 14.
In the aforesaid conventional method, the first evaporator 11 generally has to be maintained at from 110 to 120° C. Meanwhile, the NMMO solution to be fed into the first evaporator 11 also has to be heated to a relatively high temperature. Thus, a large amount of external heat is required. In addition, since the water vapor produced from the third effect evaporator 13 is required to be cooled, cooling water or a cooling device is required.